1. Field of the Invention
The present invention generally relates to intravascular medical devices used to measure blood flow velocity. More specifically, the present invention relates to intravascular diagnostic devices used to evaluate the state of coronary blood flow and velocity at basal conditions and after induced coronary hyperemia as well as the early and late outcome of coronary intervention procedures. The present invention represents a significant advance in the state of the art. Among other advantages, it offers high degrees of accuracy, safety, and speed of use, cost-to-benefit value and the potential for wider application in other cardiovascular fields.
2. Description of the Prior Art
Measurement of coronary flow velocity and coronary reserve have gained wide acceptance as crucial diagnostic values in the decision-making process for coronary angioplasty and other cardiac intervention procedures. Following the rapid progress in the quantities of angiography (QCA) achieved during the 1980s, it was hoped that anatomic information alone, enhanced by digital techniques, would become so comprehensive that there would be no further need for physiological confirmation of angiographic data. To date, however, QCA has not fulfilled its promise to predict the physiological significance of coronary artery stenosis or to quantify increases in artery flow following angioplasty.
At present, these parameters are evaluated most commonly by insertable coronary instruments such as Doppler guidewires or pressure measuring guidewires. A more recent invention describes a new guidewire that includes both pressure and flow sensors. While these methods have achieved reasonable degrees of accuracy, they all pose certain limitations whether in terms of cost, ease of use or speed in obtaining results.
Coronary flow measures have also been measured by X-ray densitometry, which is based on the mean transit time of a contrast medium between a proximal site and a distal site of the vessel. The application of this measurement technique to the coronary artery is complicated by technical problems arising from the continuous motion of the coronary artery, requiring manually positioned windows for the video-densitometric measuring device. In addition, determination of the front velocities of the contrast medium required in repeated injections by means of an ECG triggered power injector, during three to five phases of different cardiac cycles and their reconstruction to provide the flow rate pattern of a single cardiac cycle.
Thermodilution has long been a promising technique in the study of circulation. To prove its reliability in the determination of blood flow and velocity, Fegler in 1957 obtained simultaneous thermodilution curves from two catheter-mounted thermal sensors at the arch and bifurcation of the aorta by injecting room temperature saline solution into the right atrium.
More recently, Weijand et al. (U.S. Pat. No. 5,.989,192, Nov. 12, 1999) measured cardiac output by positioning a device with two closely spaced thermal sensors in the ascending aorta to detect spontaneous cyclic temperature variations during the cardiac cycle.
These spontaneous cyclic temperature variations do not extend to the coronary blood flow, as they merge and dissipate through the swirling motion of blood in the coronary sinus behind the opening aortic valve leaflet leading to vortex generation before the ostium of each coronary artery.
In addition, the mainly diastolic nature of coronary blood flow further dissipates these spontaneous temperature variations leading to a steady temperature baseline in the coronary circulation.
Examples of analogous and non-analogous prior art blood flow velocity measurement systems are disclosed in the following U.S. Patents.
However, thermodilution techniques have rarely been applied to diagnostics of the coronary arteries. The present invention represents both a significant refinement of tested thermodilution techniques, and a promising new method for measurement of coronary velocity and flow.
The present invention is a potentially cost effective intravascular guidewire system that is capable of quickly and accurately measuring coronary flow velocity and coronary reserve. The device generates these parameters by establishing the transit time of a thermal signal as it passes downstream with the coronary artery blood flow.
The proposed thermodilution guidewire includes an elongated shaft with a floppy tip that is inserted into a segment of interest in the mammalian coronary arteries for purposes of guiding an intervention catheter, scope or other medical device. The preferred embodiment includes several thermal sensors, three of which are described in the present embodiment, consisting of thermocouple measuring junctions, mounted in sequential order at equal predetermined intervals along the terminal segment of the guidewire shaft, at a distance of 10, 25 or 50 mm, proximal to its spring tip. Two respective insulated electrical paths of the same materials as the thermal junctions extend from each thermocouple in a helical winding along the length of the guidewire shaft to its proximal end, where each electrical path is joined to one of six separate sleeve electrodes. The shaft and its added components are sheathed in an insulating material suitable for smooth introduction into a human vessel.
Each of the six sleeve electrodes located at the guidewire""s proximal end is electrically connectable through external cables to its respective reference junction that is maintained at a constant temperature medium. The electrodes are also electrically connected to three separate thermocouple amplifiers, a fast sweep multiple channel color coded monitor, an online programmed computer and a printer.
When the thermodilution guidewire has been positioned at the segment of interest of the coronary artery, an upstream thermal indicator is introduced at the ostium of the coronary artery in the form of a steady, slow infusion of room temperature saline at 22 degree centigrade over 10-15 second period. This infusion is similar to the standard procedure of flushing a guiding catheter with room temperature saline during coronary interventions.
Mixing of room temperature saline infusion with coronary blood flow induces a transient temperature gradient during the period of infusion. The relatively warmer coronary flow, with its pulsatile phasic pattern of small systolic and large diastolic components, thus acts as a warm thermal indicator that reflects the degree of thermodilution. This degree changes during each phase of coronary flow, creating periodic oscillations of the temperature gradient that simulate rectified sine waves.
Each temperature oscillation is sequentially detected by the three serially mounted thermal sensors as three consecutive rectified sine waves with a phase shift between them. The degree of phase shift between consecutive waves is an expression of the transit time of blood flow between sequential thermal sensors. This value which is inversely related to the average phasic velocity is directly determined by an online computer programmed to calculate the average flow velocity from the transit time between successive phase shifts. An online multiple channel color coded monitor with adjustable sweep velocity also displays these successive oscillations during the measuring time. The monitor""s sweep velocity may be increased to match the fast average phasic velocity met with during measures of fast flow velocity of coronary reserve, and calibrated to give a real-time direct digital readout of these values.
The system can determine values for coronary flow volume when the angiographically measured diameter of the segment of interest of the coronary artery is supplied to the computer. The present invention may also determine coronary reserve, which is of particular value during decision making in cases with intermediate coronary stenosis, as well as in determining the immediate and late results of intervention procedures. For the device to determine coronary flow reserve, the value for blood flow velocity is first obtained at basal conditions, and then after inducing maximal coronary flow hyperemia. In addition, the rate of return of the trailing end of temperature drop to its original baseline level provides an indirect evaluation of transmyocardial flow velocity.
The transit-time guidewire described in the present invention may also be modified to suit the size and flow velocity of the cerebral carotid, renal and other peripheral arteries to provide valuable data on flow velocity and reserve in these vessels.